1. Field of the Invention
The present invention relates to a solid microlaser, a cavity for said microlaser and a process for the production of said cavity.
2. Discussion of the Background
The main advantage of the microlaser (cf. publications 1 and 2 listed at the end of the present description) is its structure in the form of a stack of multilayers, which constitutes its essential characteristic. The active laser medium is constituted by a material having a limited thickness of between 150 and 1000 .mu.m and of small dimensions (a few mm.sup.2), on which are directly deposited dielectric cavity mirrors. This active medium can be pumped by a III-V laser diode, which is either directly hybridized on the microlaser, or coupled to the latter by an optical fiber. The possibility of a collective manufacture using microelectronics means authorizes a mass production of said microlasers at very low cost.
Microlasers have numerous applications in fields as varied as the car industry, the environment, scientific instrumentation and telemetry.
Known microlasers generally have a continuous emission of a few dozen mW of power. However, most of the aforementioned applications require peak powers (instantaneous power) of a few kW supplied for 10.sup.-8 to 10.sup.-9 seconds, with an average power of a few dozen mW. In solid lasers, it is possible to obtain such high peak power levels by making them operate in the pulsed mode at frequencies between 10 and 10.sup.4 Hz. For this purpose use is made of well known switching processes, e.g. by Q-switch (publication 3 given at the end of the description).
More specifically, the switching of a microlaser cavity consists (publication 3) of adding thereto losses which are variable in time which will prevent the laser effect for a certain time during which the pumping energy is stored in the excited level of the gain material. These losses are suddenly reduced at precise moments, thus releasing the stored energy in a very short time (giant pulse), so that a high peak power is obtained.
In the case of a so-called active switching, the value of the losses is controlled externally by the user (e.g. a rotary cavity mirror of an intracavity electro-optical or acousto-optical type changing either the path of the beam or its polarization state). The storage time, the opening time of the cavity and the repetition rate can be chosen independently. However, this requires adapted electronics and considerably complicates the laser system.
In the case of so-called passive switching, variable losses are introduced into the cavity in the form of a material known as a saturable absorber (SA), which is highly absorbent at the laser wavelength and a low power density and which becomes virtually transparent when said density exceeds a certain threshold, which is called the saturation intensity of the SA.
In particular, passive switching using solid saturable absorbers (publications 4 and 5 ) or saturable absorbent polymers (publications 7 and 8 ) has already been performed.
The known saturable absorbers often contain organic molecules, which are responsible for the absorption. These materials are generally in liquid or plastic form and therefore frequently have a poor optical quality, very rapid aging and a poor resistance to the laser flux (publication 3).
Solid materials are also used as saturable absorbers. These solid materials are obtained by crystal growth and are doped with saturable absorbent ions such as Cr.sup.4+ (publication 5) or Er.sup.3+ (publication 6).
In certain cases, the same material (e.g. YAG) obtained by crystal growth simultaneously contains the active laser ions (e.g. Nd) and the saturable absorber ions (e.g. Cr) (publication 4).
In known lasers passively switched with the aid of these saturable absorbers, the following arrangements have been proposed for the interior of the laser cavity.
1. A first arrangement is illustrated in FIG. 1a, in which 1 represents a laser cavity, 2 the active laser material, 3 the saturable absorber and 4,5 the entrance and exit mirrors of the cavity (publication 3). There is no contact between the saturable absorber 3 on the one hand and the other elements of the cavity 1 on the other.
In this type of device, it is necessary to optically align the elements of the cavity. Moreover, optical settings may be necessary when the laser is in use.
2. In the arrangements illustrated by FIGS. 1b and 1c, a contact is ensured between the saturable absorber 3 and a mirror 4 (FIG. 1b) or the active laser material 3 (FIG. 1c) with the aid of an optical adhesive 6 (publication 7). However, the adhesive introduces a residual absorption factor and index differences at the interface between the adhesive and the adhered materials. Moreover, a possible parallelism defect between the adhered elements can also form the source of losses in the laser cavity.
3. FIGS. 1d and 1e illustrate a third possible arrangement (publication 4), where 2 represents the active laser material, but the latter is codoped with active laser ions and saturable absorber ions. Therefore the same material then serves as the active medium and the saturable absorber medium. Thus, it is impossible to independently regulate the properties of the laser material and the saturable absorber.
The thickness of the medium influences both the absorption of the saturable absorber and the absorption of the active laser ions, as well as influencing the structure of the laser modes.
Moreover, the absorption coefficients of the active laser ions and saturable absorbers are directly linked with the concentrations of said ions, which are definitively fixed during the growth of the crystals and cannot be subsequently modified. Thus, a new crystal must be produced for each laser configuration.
Finally, in the case of passively switched lasers where the same ion (e.g. Er) is used both for the laser action and as the saturable absorber, it is impossible to use this codoping method. Thus, the same ion could serve as the active ion or as the saturable absorber ion, provided that the concentrations differ very widely. For the saturable absorber, the concentration must be much higher than for the active laser material.